Command with infinite coordnates?

You're misunderstanding the purpose of the coordinates a little bit. The command as you have it will summon a primed TNT at -603 4 170 for every snowball. No matter where those snowballs are in the world, that is where the TNT will go.

What you'll want to do is use tilde notation to specify relative coordinates:

/execute @e[type=Snowball] ~ ~ ~ summon PrimedTnt

What this will do is summon primed TNT at the coordinates of every snowball entity. Be careful about putting this in a clock without some sort of /kill @e[type=Snowball] command, unless you want to have TNT summoned along the entire trajectory of the snowball.